1. Field of the Invention
The present invention relates to high efficiency heat exchange devices of the type that can exchange heat between a primary or product air stream and a secondary or working air stream subject to a minimal heat differential. Such devices can operate to provide heat recovery in combination with ventilation for domestic purposes and can also be used in evaporative cooling devices. The invention also relates to the use of such a high efficiency heat exchanger in combination with a cooling device in order to dehumidify air.
2. Description of the Related Art
Heat exchange devices of one form or another are present in virtually every device and process. The performance of an action invariably involves the release of energy in the form of heat. If not required, the heat will often be released to ambient via an appropriate heat conducting surface provided e.g. with cooling fins. If the quantity of heat is excessive or if it can be employed for useful purposes, a specific heat exchanger may be provided to transport the heat away e.g. to another system. Heat exchange may also take place between different media:—gas, liquid and solid media can be interfaced in all combinations according to the performance required. The present invention relates to heat exchangers of the air-to-air type. It is nevertheless the case that the heat is transferred from one air stream to the other by conduction through a solid medium.
Air-to-air heat exchangers are most commonly formed as membrane or plate type heat exchangers. A primary channel is separated from a secondary channel by a heat conducting plate or membrane. Primary and secondary streams of air flow through the respective channels and heat is transmitted from one stream to the other through the conducting wall. For optimum efficiency, the air streams will be arranged to flow generally opposite to one another in counter-flow. In certain situations practicality dictates that flow should take place in cross-flow, whereby one fluid flows perpendicular to the other. Generally, the heat conducting membrane will be formed of a material having good thermal conduction properties. Metals, in particular steel and aluminium may therefore be favoured. In certain situations however, materials with lower thermal conductivity may be used, subject to the thickness of the material being minimised. Since the quantity of heat transferred through a membrane is proportional to the temperature gradient across it, reducing the thickness of a membrane can quickly offset a decrease in thermal conductivity.
A problem that can arise with certain membrane and plate heat exchanger designs is the presence of unwanted heat conduction along the heat exchanger in the flow direction. This problem is significant in high efficiency heat exchangers designed to operate across a low temperature gradient. For a counter flow arrangement, heat conduction through the heat exchanger in the direction of flow leads to a reduction in the heat differential between inlet and outlet. For this reason, plastics materials have often be favoured for heat recovery devices in heating and ventilation systems.
It has also been previously proposed to install plates in a heat exchanger in such a manner that the plate itself transfers heat within its plane from the first stream to the second stream. Separation of the first and second channels is provided by separators between adjacent plates rather than by the plate itself. Since the separator no longer has a heat transmitting function it may be manufactured from an insulating material, thus reducing the cross-section for heat flow in the longitudinal direction of flow. A device of this type has been shown in JP58035387 A. Nevertheless, this principle of operation has not been generally adopted, possibly due to increased manufacturing complexity in achieving a large surface area and only limited improvement in efficiency. A further device that has attempted to improve the efficiency of heat exchange is shown in U.S. Pat. No. 5,832,992. According to that publication, a plurality of wires are arranged in mats through which air may flow. The wires are relatively closely packed together having a pitch of 1.5 to 2.5 times the wire diameter.
Another field that is closely related to heat exchange is that of (de)humidification. In the heating, cooling, ventilation and air-conditioning industries, heat exchange and dehumidification or humidification go hand in hand. Humidification is generally simpler as the increase in entropy facilitates the process. Dehumidification however requires energy and is a considerable burden on designers. Conventional dehumidifiers make use of a desiccant wheel using e.g. silica gel or the like. The desiccant absorbs moisture from an air stream passing over it. By absorbing vapour, considerable energy is released causing the air stream or the desiccant wheel to be heated. In cooling systems, this heat of absorption reduces the effectiveness of the cooling. The desiccant must also be periodically regenerated by evaporating off the absorbed moisture. This step also requires considerable energy corresponding to the latent heat of evaporation of the liquid. A desiccant wheel of this type is known from U.S. Pat. No. 5,542,968.
For air of high humidity, alternative manners of dehumidifying have also been suggested. By cooling the air to below its dew point, condensation of vapour will occur. Although the air will remain close to the level of 100% relative humidity, its absolute humidity will drop. On subsequently warming the air (in the absence of water) to its original temperature, the relative humidity will fall while absolute humidity will remain constant. The method is relatively efficient in theory but in practice requires a high efficiency heat recovery element in order to achieve the desired results. For this reason, the principle has not been widely used for dehumidification in cooling and ventilation systems. One device of this type has been described in EP0861403 A.